A conventional spark plug for an internal-combustion engine is comprised of: a center electrode in which a front end thereof serves as an electrode for spark discharge; an insulator having an axial bore and accommodating the center electrode in a front end of the axial bore; and a metal shell surrounding and holding the insulator in a radial direction thereof. A male thread portion is formed on a front end side outer surface of the metal shell so as to engage with a threaded hole of an internal-combustion engine. Then, a spark is discharged in the internal-combustion engine to thereby ignite an air-fuel mixture.
The metal shell of such a spark plug typically includes an outward projection portion disposed toward a rear end side with respect to the fitting thread portion. An annular-shaped hollow gasket is disposed on a locating portion formed between the seal portion and the fitting thread portion. The hollow gasket is sandwiched between the surface of the engine block that surrounds the threaded hole therein and the seal portion of the metal shell when the spark plug is mounted on an internal-combustion engine. The hollow gasket is deformed to thereby improve its sealing properties and prevent the air leakage of the combustion chamber through the fitting threaded hole. Such a hollow gasket is produced by, for example, radially bending a ring-like plate member into an “S” shaped or a “C” shaped cross-section. As a result, the gasket is easily deformed when mounting the spark plug, and its sealing properties may be maintained after being deformed.
In the manufacturing process of the spark plug, the hollow gasket is inserted from the front end side of the metal shell having a thread ridge in the fitting thread portion, which is subjected to a cutting process, and is disposed on the locating portion. At this time, plural parts of an inner edge of the hollow gasket is compressed in an axial direction so as to form a nail-like portion, which radially inwardly projects with respect to a portion serving as the maximum outer diameter of the fitting thread portion. As a result, the gasket is retained on the metal shell and is prevented from falling from the metal shell over the fitting thread portion (e.g., refer to Patent Document 1).
In recent years, the improvement in an output of an automobile engine and fuel efficiency are highly in demand, and further improvement in spark plug components is also required. Regarding a gasket, a flat solid gasket assuming a thick disc shape and comprised of an alloy, which is mainly made of copper or the like, is considered. One of the advantages of using such a flat solid gasket is that after being once mounted on the engine, the spark plug is unlikely to loosen. Further, because such a gasket is a flat solid member, it is unlikely to be crushed. Furthermore, the position of the front end of the center electrode within the combustion chamber, with respect to an axis of the fitting threaded hole, is unlikely to vary, thereby stabilizing an igniting position.
In the manufacturing process of the spark plug using the flat solid gasket, in order to prevent the flat solid gasket from falling out from the metal shell, for example, a pipe shaped pressing member having an inner diameter slightly lager than the maximum outer diameter of the thread ridge is provided from the front end side of the metal shell in a state that the flat solid gasket is disposed on the locating portion of the metal shell in which the fitting thread portion has already been formed. Further, a front end opening of the pressing member being in contact with the flat solid gasket is pressed against the seal portion. As a result, the inner edge of the flat solid gasket radially inwardly projects from a portion serving as the maximum outer diameter of the fitting thread portion.
However, in the manufacturing process of a spark plug, since there is no large difference in diameters between an inner circumference of a pressing member and a thread ridge of a fitting thread portion of the metal shell when disposing a gasket on an locating portion of a metal shell after a thread rolling and processing for preventing a falling out of a gasket, and there is a tendency of producing a chip of the thread ridge. In order to prevent a loosening of a spark plug, the hardness of the gasket is necessary to be raised. However, when such a gasket is used, a pressing force against the gasket using a pressing member during a process of preventing the falling out of the gasket needs to be increased. As a result, a durability of a pressing member decrease, thereby causing a rise of a production cost.
An advantage of the present invention is a method for manufacturing a spark plug, and a spark plug manufactured by the method, wherein a gasket used for securing the air-tightness between the spark plug and an internal-combustion engine is prevented from falling from the metal shell of the spark plug with a simple step.